Chapter 4 INTRUSIVE IGNEOUS ROCKS Polymerization of Silica Each Si atom is surrounded by 4 oxygens Polymerization is the degree to which the oxygens are shared with other Si atoms Polymerization is the number of Si O Si bonds Olivine has none isolated SiO4 groups Pyroxene has one third SiO3 chains Mica has has 40 shared Si4O10 sheets Quartz and feldspar have 100 Si Al O2 framework Polymerization of Silica Pyroxene Tetrahedron Olivine Polymerization of Silica Mica Sheet Quartz Framework MAGMA Magma is the term for any molten silicate material whether below the surface or on top Volcanic rocks are erupted on the surface Volcanic rocks are fine grained 1 mm Intrusive igneous rocks crystallize from magma below the surface Intrusive igneous rocks are coarse grained 1mm Pegmatites are very coarse grained 1cm Intrusive Igneous Rocks Composition Mineralogy Geologic Setting Mantle Oceanic Crust Continental Crust Igneous Fractionation How the chemistry evolves Igneous Rock Compositions Rock compositions are described in weight percents of oxides The principal variation in igneous rock compositions is silica SiO2 content The degree of polymerization of silica increases with silica content both in crystals and melt Igneous Rock Compositions Igneous rocks vary in composition from ultramafic 40 wt peridotite mafic 48 55 gabbro intermediate 55 65 diorite silicic 65 75 wt granite The mantle is peridotite The ocean basins gabbro The continents are granite Igneous Rock Names and Compositions Intrusive Volcanic Compositions Rock Name Rock Name Ultramafic Mafic Intermediate Silicic felsic Peridotite Gabbro Diorite Granite Komatiite Basalt Andesite Rhyolite Igneous Rock Mineralogy Perodotite Ultramafic Gabbro Mafic Olivine Mg Fe 2SiO4 Pyroxene Mg Fe Ca SiO3 Garnet Mg3Al2Si3O12 Spinel MgAl2O4 Feldspar CaAl2Si2O8 Pyroxene Mg Fe Ca SiO3 Olivine Mg Fe 2SiO4 Igneous Rock Mineralogy Diorite Intermediate Feldspar CaAl2Si2O8 Feldspar NaAlSi3O8 Pyroxene Mg Fe Ca SiO3 Mica K Mg Fe 3 Si Al 2O10 OH 2 Granite Silicic Quartz SiO2 Feldspar NaAlSi3O8 Feldspar KAlSi3O8 Mica biotite K Mg Fe 3 Si Al 4O10 OH 2 Mica muscovite KAl2 Si Al 4O10 OH 2 Geologic Settings Compositions Where Ultramafic Mafic Intermediate Silicic felsic Mantle Oceanic Continental Mtns Continental Mass Igneous Fractionation Peridotite basalt granite Partial Melting Rocks have a melting interval of several hundred degrees C First melt is rich in silica water and trace elements Fractional Crystallization First formed crystals low in silica settle to the bottom Intrusive forms Pluton Any large discordant intrusion at depth Batholith large 100 km2 Stock small 100 km2 Dike Tabular non conformable Sill Tabular conformable
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